Method of production of steel product with nanocrystallized surface layer

ABSTRACT

A method of production of a metallic product with a nanocrystallized surface layer comprising subjecting a surface layer of a metallic product to ultrasonic impact treatment by one or more ultrasonic indenters vibrating in a plurality of directions, then subjecting the surface layer subjected to the ultrasonic impact treatment to heat treatment at a low temperature to cause precipitation of nanocrystals.

TECHNICAL FIELD

The present invention relates to a method of production of a metallicproduct with a nanocrystallized surface layer.

BACKGROUND ART

Metallic products are superior in strength and cost compared with othermaterials, so are being used in a variety of fields such as offshorestructures, ships, bridges, automobiles, industrial machinery, householdelectrical appliances, medical equipment, etc. Therefore, metallicproducts play important roles in industry.

However, the ultrahigh strength, fatigue resistance, wear resistance,and other characteristics required for metallic products are importantcharacteristics not for the metallic products as a bulk, but inparticular for the surface layers of the metallic products. In manycases, there is no need for the products as a bulk to have suchcharacteristics.

Therefore, broad use is being made of the method of controlling thecrystal structure of the surface layer of a metallic material so as toimpart various superior properties to the material. Up to now, asuccession of superior materials have been created with the introductionof each new process for the control of the crystal structure. In thefuture as well, there is a possibility of much more superior materialsbeing created due to the introduction of new processes.

In recent years, it has become possible to refine the crystal structuresof metallic materials to a nanometer (nm, 10⁻⁹ m) level size (forexample refined to less than 100 nm), i.e., to achieve a nanocrystalstructure, so as to obtain superior properties not achievable in thepast, for example, ultrahigh strength.

As a method of obtaining a metallic material having a nanocrystalstructure, there is known the method of once amorphize the metallicmaterial and then converting it from a amorphous state to a crystallinestate so as to obtain a nanocrystal structure.

As a method of amorphizing a metallic material, the method of high speedrapid cooling of the melt of the metallic material, sputter deposition,or other methods may be used.

If making the atomic configuration of a metallic material amorphous,unique properties not obtainable by a crystallined metal are obtainedand a metallic material having high strength, corrosion resistance, highmagnetic permeability, and other superior properties can be obtained.

By heat treating such an amorphous metallic material at a lowtemperature, it is possible to make fine nanometer (nm, 10⁻⁹ m) sizecrystals, that is, nanocrystals, precipitate. Further, it is possible toobtain a metallic material exhibiting properties more superior to anamorphous metal, for example, a metallic material exhibiting ultrahighstrength or a metallic material superior in magnetic characteristics(for example, see Japanese Unexamined Patent Publication (Kokai) No.1-110707 or Japanese Patent No. 1944370).

The method of amorphizing a metallic material and then heat treating itat a low temperature to cause nanocrystals to precipitate in this wayshould be taken note of as a method for imparting superior propertiesand functions not achievable with conventional methods to a metallicmaterial.

However, in providing metallic materials using this method for actualuse, there have been the problems explained below.

First, as methods for obtaining metallic materials in the amorphousstate, there are the method of high speed rapid cooling of the melt ofthe metallic material and the method of sputter deposition, but thesemethods involve high speed rapid cooling or deposition, so there aremajor restrictions on the shape or dimensions, and application to theproduction of shaped articles, structures, and metallic products ofgeneral shapes has been difficult.

Further, as the method of amorphizing a metallic material and causingnanocrystals to precipitate, in addition to the above-mentioned methods,the following method is known.

That is, it is possible to treat a powder of a metallic material by aball mill etc., then work-harden the surface of the material toamorphize the material, then heat treat the material to obtain ametallic material with nanocrystals precipitated.

The thus produced metal powder may be used not only as an alloy powderof an amorphous metal as it is, but may also be press formed and used asshaped articles, structures, and metallic products of general shapes.

It becomes necessary to press form this powder at a high temperature toobtain a shaped article having sufficient strength for this purpose orweld such shaped articles to fabricate a desired structure.

However, if an alloy powder of an amorphous metal experiences a hightemperature process, the powder will lose its nanocrystal structure andchange to a large crystal structure. Therefore, it was not possible toobtain a shaped article, structure, or metallic product making use ofthe features of a nanocrystalline structure from a metal powder withnanocrystals precipitated.

Note that for example the specification of U.S. Pat. No. 6,171,415discloses a method of modification of the fatigue strength by applyingultrasonic vibration to a welded joint zone, but does not discloseapplying ultrasonic vibration to the surface layer of a metallic productto make it nanocrystalline.

SUMMARY OF INVENTION

The present invention has as its object to solve the above-mentionedproblems of the prior art and provide a method of production of ametallic product with a nanocrystallized surface layer.

The present invention was made as a result of intensive study forsolving the above problems and provides a method of production of ametallic product with a nanocrystallized surface layer madenanocrystalline by subjecting the surface layer of the metallic productto ultrasonic impact treatment for impacting by an ultrasonic indenterso as to work-harden the surface layer, then heat treating this at a lowtemperature.

Further, the gist is as follows:

(1) A method of production of a metallic product with a nanocrystallizedsurface layer, the method of production of a metallic product with ananocrystallized surface layer characterized by comprising (1)subjecting a surface layer of a metallic product to ultrasonic impacttreatment impacting it by one or more ultrasonic indenters vibrating ina plurality of directions, then (2) subjecting the surface layersubjected to the ultrasonic impact treatment to heat treatment at a lowtemperature to cause precipitation of nanocrystals.

In the present invention, the “metallic product” includes not onlybridges, buildings, and other so-called steel structures, but also themetallic parts, steel plates, aluminum products, titanium products, andother common products made of metal.

Further, the “nanocrystal” means fine crystals of a nanometer size, thatis, a 10⁻⁹ m size. The range of the grain size is, from the propertiesshown, an average grain size of 1 to 100 nm, more preferably 3 to 30 nm.

(2) A method of production of a metallic product with a nanocrystallizedsurface layer as set forth in (1), characterized in that the surfacelayer of the metallic product subjected to the ultrasonic impacttreatment is in an amorphous state.

(3) A method of production of a metallic product with a nanocrystallizedsurface layer as set forth in (1) or (2), characterized in that theultrasonic impact treatment is accompanied with mechanical alloying.

(4) A method of production of a metallic product with a nanocrystallizedsurface layer as set forth in any one of (1) to (3), characterized bymaking an amorphous phase and a nanocrystal phase copresent inprecipitation of the nanocrystals.

(5) A method of production of a metallic product with a nanocrystallizedsurface layer as set forth in any one of (1) to (4), characterized byshielding the surroundings at the time of the ultrasonic impacttreatment from the air.

(6) A method of production of a metallic product with a nanocrystallizedsurface layer as set forth in any one of (1) to (5), characterized inthat the surface layer of the metallic product is comprised of a ferrousmetal and the surface layer is subjected to heat treatment for heatingat 100 to 500° C. for 15 minutes or more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view of a first embodiment of the present invention.

FIG. 2 is a plan view seen along the line X-X′ of FIG. 1.

FIG. 3 is a view illustrating vibration waveforms of indenters of A, B,and C shown in FIG. 1.

FIG. 4 is a view of a second embodiment of the present invention.

THE MOST PREFERRED EMBODIMENT

The embodiments of the present invention will be explained in detailusing FIG. 1 to FIG. 4.

<First Embodiment>

In FIG. 1, 1 indicates an ultrasonic vibration apparatus, 2 ultrasonicindenters, and 3 a shield gas feed apparatus.

First, as shown in FIG. 1, the surface layer of a metallic product isimpacted by the ultrasonic indenters 2.

In the present embodiment, a plurality of (three) ultrasonic indenters 2is provided. The tips of the indenters are made to vibrate in differentdirections (in the figure, Z₁, Z₂, and Z₃).

The reason for impacting the surface layer of the metallic product byone or more ultrasonic indenters vibrating in a plurality of directionsis as follows:

In working by impacting making ultrasonic indenters vibrate in only onedirection, the structure of the surface layer of the metallic product isdeveloped, the crystal grains do not become equiaxial, and the crystalgrains deform to pancake shapes. High angle grain boundaries are notformed.

Therefore, by using a plurality of ultrasonic indenters, making the tipsof the ultrasonic indenters vibrate in a plurality of differentdirections, and impacting the surface layer of the metallic product,formation of texture is suppressed and the grains become equiaxial.

Further, by heat treating at a low temperature the surface layer of themetallic product subjected to the ultrasonic impact treatment, it ispossible to make the surface layer nanocrystalline.

This ultrasonic impact treatment work-hardens the surface layer of themetallic product in a range of for example a surface layer of 100 μm soas to sufficiently disarrange the crystal lattice and cause the loss ofthe properties as crystals and for example form a state of atomicconfiguration disarranged to an extent not allowing movement ofdislocations at the surface layer.

Further, to facilitate nanocrystallization, it is preferable to useultrasonic impact treatment to make the surface layer of the metallicproduct, for example, the range of a 100 μm surface layer, an amorphousstate with no long period atomic configuration.

The ultrasonic impact treatment is performed cold. If performing it notcold, but at the recrystallization temperature or a higher temperature,the work-hardening causes the recrystallization of the layer with adisarranged crystal lattice to proceed rapidly resulting in crystals ofa large grain size and difficulty in obtaining a nanocrystal structure.

Therefore, the temperature of the ultrasonic impact treatment has to bemade a temperature sufficiently lower than the recrystallizationtemperature of the metallic material.

The ultrasonic impact treatment is accompanied with the heat of workinggenerated, so when necessary the surface layer of the metallic productis cooled so that the temperature of the surface layer is brought closerto the recrystallization temperature.

In the present invention, the angles of the plurality of vibrationdirections are not limited, but the impact is applied from as differentdirections as possible. Therefore, as shown in FIG. 1, it is preferableto make the incident angle (θ) with respect to the surface layer of themetallic product 30 degrees or more.

After the ultrasonic impact treatment, the surface layer is heat treatedat a low temperature to cause precipitation of nanocrystals. This heattreatment is performed at a low temperature at which the crystal grainswill not coarsen.

As the heat treatment temperature, a temperature higher than the ambienttemperature at which the metallic product is used is selected. If usinga Cooper heater etc. for heat treatment over a sufficient time, it ispossible to obtain stable nanocrystals at the surface layer of themetallic product.

In the present invention, the size of the crystal grains forming thenanocrystal structure can be suitably selected in accordance with thecomposition of the metallic material or the object, but in averagediameter is 1 to 100 nm, more preferably 3 to 30 nm.

The shield gas feed apparatus 3 blows argon, helium, CO₂, or anotherinert gas to the tips of the ultrasonic indenters to shield thesurroundings at the time of the ultrasonic impact treatment from theair. The action and effect of this will be explained later.

Note that the heat treatment when the metallic product is comprised of aferrous material is preferably performed suitably selecting the surfacetemperature in the range of 100 to 500° C. and the treatment time in therange of 15 minutes or more considering the ease of recrystallization offerrous materials.

FIG. 2 is a plan view seen along line X-X′ in FIG. 1 showing a firstembodiment.

In FIG. 2, the ultrasonic indenters 2 are arranged at angles of 120degrees from each other and are structured so that the tips of theultrasonic indenters are made to vibrate in different directions.

FIG. 3 is a view of the vibration waveforms of the indenters of A, B,and C shown in FIG. 1.

In FIG. 3, the vibration waveforms (F) of A, B, and C are offset by ⅓ aperiod each to make the tips of the vibration indenters 2 vibrate insuccessively different directions, so the structure of the surface layerof the metallic product can be efficiently made nanocrystalline.

<Second Embodiment>

In FIG. 4, 1 indicates ultrasonic vibration apparatuses and 2 ultrasonicindenters.

In the present embodiment, a plurality of ultrasonic indenters 2 areused bundled together. The bundled ultrasonic indenters 2 as a bulk aresimultaneously made to vibrate in the vertical direction (Z₄) and thehorizontal direction (Z₅). Therefore, a plurality of ultrasonicvibration apparatuses 1 are provided.

By making the ultrasonic indenters 2 vibrate simultaneously in thevertical direction and horizontal direction and impact the surface layerof the metallic product, it is possible to suppress the formation oftexture and make the crystal grains equiaxial.

Further, after this, it is possible to heat treat the surface layer ofthe metallic product at a low temperature to cause the precipitation ofnanocrystals and make the surface layer nanocrystalline.

Note that even if using a single ultrasonic indenter 2 and making itvibrate in the vertical direction or even if making the ultrasonicindenters turn or rock instead of vibrating in the horizontal direction,it is possible to obtain similar effects.

<Embodiments Common to First Embodiment and Second Embodiment>

The inventors discovered that if nitrogen enters at the time ofsubjecting the surface layer of the metallic product to ultrasonicimpact treatment, a Cottrell atmosphere is formed and the strengthrises, but the toughness sometimes falls, so this is not preferable.

Further, the inventors discovered that if performing the ultrasonicimpact treatment in the air, the metal of the surface layer of themetallic product reacts with the oxygen in the air whereby an oxidelayer ends up being formed and that even with nanocrystallization, thepredetermined functions cannot be obtained in some cases. That is, theinventors discovered that the minimization of the oxide layer isessential.

Therefore, to secure the thickness of the nanocrystallized layer andsuppress the thickness of the oxide layer to a minimum, it is preferableto shield the surroundings at the time of ultrasonic impact treatmentfrom the air. That is, by shielding from the oxygen, the oxidation ofthe surface is prevented.

In the present invention, the method of shielding the surroundings isnot limited, but it is preferable to blow argon, helium, CO₂, or anotherinert gas at the tips of the ultrasonic indenters so as to control theenvironment to an oxygen partial pressure lower than that of air.

Due to this, the oxide layer is eliminated and the phenomenon ofembrittlement due to nitrogen penetration can be prevented.

In the precipitation of the nanocrystals, it is possible to causeprecipitation of nanocrystals without leaving any work-hardened phase orpossible to cause copresence of the work-hardened phase, for example,the amorphous phase, and the nanocrystal phase. By causing thecopresence of the amorphous phase and nanocrystal phase, it is possibleto increase the strength of the material or maintain a high corrosionresistance.

In this case, to obtain the effect of the nanocrystal structure, it ispreferable to make the ratio by volume of the crystal phase to theamorphous phase at least 15 to 85. Further, to obtain the effect ofcopresence of the crystal phase and amorphous phase explained above, itis preferable to make the ratio of volume of the crystal phase to theamorphous phase not more than 80 to 20.

In the present invention, the ultrasonic impact treatment may beaccompanied with mechanical alloying.

For example, it is possible to have the ultrasonic indenters and thesurface layer of the metallic product plastically deform with each otherto cause mechanical alloying between them.

By properly selecting the composition of the material of the ultrasonicindenters and making the surface layer of the metallic product in theamorphous state obtained along with mechanical alloying a nanocrystalstructure, it is possible to obtain a nanocrystal structure of a desiredalloy composition or give a desired composition to the vicinity of thenanocrystals.

In this way, by amorphizing the surface layer of the metallic productand simultaneously causing mechanical alloying in ultrasonic impacttreatment, it is possible to obtain a nanocrystallized metallic producthaving more superior characteristics.

According to the present invention, it is possible to finally work orassemble the steel structure, steel product, or other metallic product,then make the surface layer nanocrystalline, so it is possible to keepapplication of the present invention to the minimum necessary extent.

Further, it is possible to apply the present invention at the materialstage, finally work or assemble the steel structure, steel product, orother metallic product, then repair a region damaged by the working orassembly by again applying the present invention to just that region.

Note that the present invention may be locally applied to a region ofthe metallic product for which modification by nanocrystallization isdesired or may be applied to the metallic product as a whole.

When applying the present invention to the metallic product as a bulk,it is preferable to subject the steel plate or other material formingthe metallic product to the ultrasonic impact treatment of the presentinvention in advance and produce the metallic product using a materialwith a nanocrystallized surface layer.

The ultrasonic wave generation apparatus used for the present inventionis not particularly limited in type, but an apparatus which uses a 2 Wto 3 kW ultrasonic wave generation source, uses a transducer togenerated a 2 kHz to 60 kHz ultrasonic vibration, and uses a waveguideto amplify it and cause ultrasonic indenters provided with one or moreof 1 mm to 5 mm diameter pins to vibrate by an amplitude of 20 to 60 μmis preferable.

However, the tips of the ultrasonic indenters in the first embodimentreceive vibration from a plurality of ultrasonic indenters, so arepreferably round with diameters of at least 10 mm.

Above, by using the present invention, it is possible to obtain ametallic product with a surface part given an ultrahigh strength andexcellent toughness.

An experiment was conducted envisioning application of the presentinvention to actual metallic products. The results are shown in Table 1to Table 4.

Table 1 shows the chemical compositions (mass %) and thicknesses (mm) ofthe materials A (A1 to A13) forming metallic parts.

Table 2 shows the ultrasonic impact treatment conditions and heattreatment conditions, while Table 3 (continuation of Table 2) shows thetest results.

* 1). <Type of Working>

The type of working, as shown in Table 4, is use of round-tip pins asultrasonic indenters.

* 2) <Thickness of Modified Layer>

The thickness of the modified layer shows the thickness from the surfaceof the layer where the microstructure of the metallic product changes tobecome amorphous or finer in crystal grains.

* 3) <Nanocrystallization Ratio (%)>

The nanocrystallization ratio shows the area ratio (%) of the region inthe modified layer where the crystal grain size can be determined withan electron microscope to be less than 1 μm.

<Amorphous Ratio (%)>

The amorphous ratio shows the area ratio (%) of the region in themodified layer where crystal grains cannot be observed with an electronmicroscope.

* 4) <Hardness Ratio Before/After Modification of Surface Layer>

The hardness ratio before/after modification of the surface layer showsthe ratio of the hardness of the surface layer of the metallic partafter application of the present invention to the hardness beforeapplication of the present invention.

* 5) <Results of Fatigue Test by Micro Test Piece>

The region including the layer modified by ultrasonic impact treatmentwas observed by a scanning electron microscope and a test piece was cutout from that region by ion sputtering.

A micro test piece of a thickness of 20 μm, a width of 100 μm, and alength of 800 μm was used for a fatigue test by a microtester system soas to find an S—N diagram.

Further, the fatigue strength indicating fracture at 1,000,000 cycleswas evaluated by the ratio of modification of the fatigue strengthbefore/after modification as defined by the following equation:

Ratio of modification of fatigue strength before/aftermodification=(Fatigue strength of 1,000,000 cycles at modifiedlayer)/(Fatigue strength of 1,000,000 cycles at test piece taken fromunmodified region)

-   -   * 6) <Results of Evaluation of Corrosion Loss by Micro Test        Piece>

The region including the layer modified by ultrasonic impact treatmentwas observed by a scanning electron microscope and a test piece was cutout from that region by ion sputtering.

A micro test piece of a thickness of 20 μm, a width of 100 μm, and alength of 800 μm was used for a salt water spray corrosion test. Theresults of the corrosion test are affected by the corrosion conditionsand the corrosion sensitivity of the material, so an unambiguousinterpretation of the results is extremely difficult.

Therefore, a micro test piece taken from an unmodified region and amicro test piece taken from the modified layer were simultaneouslysubjected to a corrosion test under the same conditions and the changein the weight loss due to corrosion over time was measured.

When the corrosion loss of the test piece taken from the region not themodified layer became 30%, the corrosion loss of the test piece takenfrom the modified layer was measured and the ratio was evaluated by theratio of modification of the corrosion loss before/after modificationdefined by the following equation:

Ratio of modification of corrosion loss before/aftermodification=(Corrosion loss at modified surface)/(Corrosion loss attest piece taken from non-modified region)

No. 1 to No. 18 are examples of the invention satisfying the conditionsof the present invention. According to these examples of the invention,it was confirmed that by applying the present invention to a steelstructure, steel part, steel plate, aluminum product, titanium product,or other metallic product, it is possible to remarkably improve the wearresistance, fatigue resistance, and corrosion resistance.

TABLE 1 No. of Material Matrix Chemical composition (mass %) A Materialcomponent C Si Mn P S Al Ti A1 Steel Fe 0.10 0.26 1.18 0.006 0.003 0.0260.009 A2 Steel Fe 0.08 0.21 1.46 0.008 0.003 0.021 0.010 A3 Steel Fe0.06 0.27 1.38 0.006 0.004 0.011 0.008 A4 Steel Fe 0.04 0.18 1.44 0.0090.005 0.022 0.015 A5 Steel Fe 0.07 0.25 1.30 0.007 0.003 0.015 0.014 A6Steel Fe 0.04 0.11 0.92 0.009 0.005 0.022 0.015 A7 Steel Fe 0.27 0.251.41 0.006 0.003 0.029 (wear resistant steel) A8 Steel Fe 0.06 0.80 0.180.002 0.002 (stainless steel) A9 Steel Fe 0.09 0.24 0.55 0.005 0.0030.075 (heat resistant steel) A10 Aluminum Al 0.30 0.61 Bal. alloy A11Titanium Ti 2.20 2.100 Bal. alloy A12 Magnesium Mg 0.12 2.900 alloy A13Ni super Ni 0.05 0.40 0.50 0.750 alloy No. of Thick- Material Chemicalcomposition (mass %) ness t A Ni Cu Mg Mo Cr Nb V B (mm) A1 0 0.02 0.1225 A2 0.0004 0.02 0.0016 60 A3 0.41 0.40 0 0.004 0.05 70 A4 0.14 0.150.0002 0.3 0.2 0.01 0.2 70 A5 0.0017 0.02 0.1 40 A6 1.50 0.0002 0.3 0.20.01 0.2 70 A7 0.52 0.0012 30 A8 10.00 19 20 A9 10.20 1 9.02 0.07 0.2 20A10 0.55 1.6000 0.05 Zn: 0.2 20 A11 15 A12 0.10 0.01 Bal. Zn: 1.1 35 A13Bal. 0.05 15 0.9 Fe: 7.0 20

TABLE 2 Heat treatment after Ultrasonic impact treatment working Inv.Treatment Temp. during Heat ex. Material Type of Output Frequency timetreatment at treatment Treatment no. Application A working Atmosphere(W) (kHz) (min) surface layer (° C.) temp. (° C.) time (min) 1 Steelstructures A1 H(1) CO₂ gas 1000 40 3 50 200 600 2 Steel structures A1H(1) Air 500 60 3 45 240 20 3 Steel plate A2 H(1) CO₂ gas 200 20 10 90450 30 4 Steel structures A2 H(2) Argon gas 1000 10 2 120 200 70 5 Steelstructures A3 H(2) Argon gas 1000 2 1 200 100 20 6 Parts A4 H(2) Argongas 500 40 3 90 300 14 7 Parts A5 H(2) Helium gas 2 60 20 90 500 5 8Steel plate A6 H(2) Air 200 20 2 70 230 35 9 Aluminum products A7 H(2)CO₂ gas 1000 10 4 40 150 70 10 Titanium products A8 H(1) Argon gas 500 25 35 300 50 11 Mg products A12 H(3) Argon gas 200 60 2 200 100 40 12 Niproducts A13 H(3) Helium gas 2 20 30 40 350 5 13 Steel structures A1H(1) Argon gas 1000 40 3 130 100 40 14 Steel structures A1 H(1) Argongas 500 60 3 45 400 8 15 Steel plate A2 H(1) Helium gas 200 20 10 90 5003 16 Steel structures A2 H(1) Helium gas 1000 10 2 200 550 35 17 Steelstructures A3 H(1) Helium gas 1000 2 1 150 450 70 18 Parts A4 H(1)Helium gas 500 40 3 300 100 20

TABLE 3 Properties after working Hardness ratio Nanocrystal-before/after Results of Inv. Thickness of lization Amorphousmodification of fatigue test by Results of evaluation Characteristics ofex. modified ratio (%) ratio (%) surface layer micro test of corrosionloss by surface layer no. layer (μm) (*3) (*3) (*4) piece (*5) microtest piece (*6) (expected function) 1 1200 85 15 3.6 3.158 1.00 Wearresistance, fatigue resistance 2 450 75 25 3.2 2.76 0.71 Corrosionresistance, fatigue resistance 3 200 65 35 2.6 2.373 0.56 Corrosionresistance, fatigue resistance 4 3400 20 80 1 0.78 0.28 Corrosionresistance 5 2100 15 85 0.8 0.618 0.26 Corrosion resistance 6 700 85 153.6 3.158 1.00 Wear resistance, fatigue resistance 7 32 90 10 3.8 3.3631.00 Wear resistance, fatigue resistance 8 200 25 75 1.2 0.945 0.29Corrosion resistance 9 3200 75 25 3.2 2.76 0.71 Wear resistance, fatigueresistance 10 1200 80 20 3.4 2.958 0.83 Wear resistance, fatigueresistance 11 300 80 20 3.4 2.958 0.83 Wear resistance, fatigueresistance 12 25 75 25 3.2 2.76 0.71 Wear resistance, fatigue resistance13 2500 80 20 3.4 2.958 0.83 Wear resistance, fatigue resistance 14 2580 20 3.4 2.958 0.83 Wear resistance, fatigue resistance 15 1200 75 253.2 2.76 0.71 Wear resistance, fatigue resistance 16 210 25 75 1.2 0.9450.29 Corrosion resistance 17 1300 70 30 3 2.585 0.63 Wear resistance,fatigue resistance 18 700 20 80 1 0.78 0.28 Corrosion resistance

TABLE 4 Type of multiaxis Type Indenter tip Shape of tip working H(1)Pin Round FIG. 1, 2 type H(2) Pin Round FIG. 4 type H(3) Pin RoundRotating pin

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a metallicproduct with a nanocrystallized surface layer. Therefore, the presentinvention provides an industrially useful metallic product.

1. A method of production of a steel product with a nanocrystallizedsurface layer, said method comprising the steps of: (1) subjecting asurface layer of a steel product to ultrasonic impact treatment byimpacting it at a plurality of different directions and angles using oneor more ultrasonic indenters, wherein said one or more indenterscomprise three indenters joined at their tips such that the tips of theultrasonic indenters vibrate in a plurality of different directions,with said ultrasonic impact treatment of said surface layer providingequiaxial grains in said surface layer, then, (2) subjecting the surfacesubjected to the ultrasonic impact treatment to heat treatment at 100°to 500° C. for 15 minutes or more to cause precipitation ofnanocrystals.
 2. A method of production of a steel product with ananocrystallized surface layer as set forth in claim 1, characterized inthat said ultrasonic impact treatment produces an amorphous state insaid surface layer.
 3. A method of production of a steel product with ananocrystallized surface layer as set forth in claim 1, characterized inthat said ultrasonic impact treatment is accompanied with mechanicalalloying.
 4. A method of production of a steel product with ananocrystallized surface layer as set forth in claim 1, characterized bymaking an amorphous phase and a nanocrystal phase copresent inprecipitation of said nanocrystals.
 5. A method of production of a steelproduct with a nanocrystallized surface layer as set forth in claim 1,characterized by shielding the surroundings at the time of saidultrasonic impact treatment from the air.
 6. A method of production of asteel product with a nanocrystallized surface layer as set forth inclaim 1, wherein at least one of the indenters is arranged to provide anincident angle with respect to the surface layer of the steel product of30 degrees or more.
 7. A method of production of a steel product with ananocrystallized surface layer as set forth in claim 1, wherein thethree indenters are arranged at 120 degrees from each other.
 8. A methodof production of a steel product with a nanocrystallized surface layeras set forth in claim 6, wherein vibration waveforms of the indentersare offset by ⅓ period from each other.
 9. A method of production of asteel product with a nanocrystallized surface layer as set forth inclaim 1, wherein said ultrasonic impact treatment is by impacting saidsurface at a plurality of different directions using one indenter madeto turn or rock.
 10. A method of production of a steel product with ananocrystallized surface layer as set forth in claim 1, wherein atemperature of the ultrasonic impact treatment is made to be atemperature lower than the recrystallization temperature of the steel.11. A method of production of a steel product with a nanocrystallizedsurface layer, said method comprising the steps of: (1) subjecting asurface layer of a steel product to ultrasonic impact treatment byimpacting it at a plurality of different directions using one or moreultrasonic indenters made to simultaneously vibrate in the verticaldirection and the horizontal direction with said ultrasonic impacttreatment of said surface layer providing equiaxial grains in saidsurface layer, then, (2) subjecting the surface subjected to theultrasonic impact treatment to heat treatment at 100° to 500° C. for 15minutes or more to cause precipitation of nanocrystals.
 12. A method ofproduction of a steel product with a nanocrystallized surface layer asset forth in claim 11, characterized in that said ultrasonic impacttreatment produces an amorphous state in said surface layer.
 13. Amethod of production of a steel product with a nanocrystallized surfacelayer as set forth in claim 11, characterized in that said ultrasonicimpact treatment is accompanied with mechanical alloying.
 14. A methodof production of a steel product with a nanocrystallized surface layeras set forth in claim 11, characterized by making an amorphous phase anda nanocrystal phase copresent in precipitation of said nanocrystals. 15.A method of production of a steel product with a nanocrystallizedsurface layer as set forth in claim 11, characterized by shielding thesurroundings at the time of said ultrasonic impact treatment from air.16. A method of production of a steel product with a nanocrystallizedsurface layer as set forth in claim 11, wherein said ultrasonic impacttreatment is by impacting said surface at a plurality of differentdirections using one indenter made to turn or rock.
 17. A method ofproduction of a steel product with a nanocrystallized surface layer asset forth in claim 11, wherein a temperature of the ultrasonic impacttreatment is made to be a temperature lower than the recrystallizationtemperature of the steel.